The goal is to identify, clone, sequence, and characterize two genes on the human X chromosome that control mineral homeostasis in children. Although the function of the encoded proteins remains to be determined, our clinical studies of patients with defects linked to the two loci of interest show that one is essential for parathyroid gland development and the other controls phosphate homeostasis. Specifically, the defects cause X-linked recessive idiopathic hypoparathyroidism (HPT) and X-linked hypophosphatenia (HYP). We have mapped the disease loci in affected families, including unique resources in local populations, and the corresponding genes are present in overlapping YAC clones that are on hand. The specific aims of the proposal focus first on the isolation of candidate genes for the two diseases from YACs in which they are localized. Several protocols will be used, including a new recombination- based method that truncates a YAC at sites homologous to cDNA sequences, and inference of candidate genes based on targeted sequencing of CpG islands and long tracts of genomic DNA harboring the loci. Following the isolation of candidate genes, studies would turn to identification of the true candidates, with accompanying analysis of the mutational changes in affected individuals. These studies would serve both as a route to correlate genotypic changes with the severity and pathophysiology of the disease, and as a prelude to characterization of the protein products and the effects of gene modification or disruption.